Java reflection API: Invoking a method without parameters

Question

The method I want to invoke (I know it's public but I need to use reflection):

public byte[] myMethod()

I get the Method object like this and m contains myMethod() (I checked with the debugger)

Method m = Class.forName(MyClass.class.getName()).getDeclaredMethod("myMethod");

Finally I need to invoke m and pass the result to an object:

byte[] myBytes = null;
m.invoke(myBytes);

No exception is thrown but myBytes stays null... I also tried the following without more success:

m.invoke(myBytes, (Object[])null);

How can I get the result of the invocation to myBytes?

T.J. Crowder · Accepted Answer · 2013-01-21 17:16:56Z

36

No exception is thrown but myBytes stays null

Correct, what you wanted there was:

byte[] myBytes = (byte[])m.invoke(yourInstance);

More in the documentation. Notes:

The return value of the method is the return value of invoke.
The first argument to invoke is the instance on which to call the method (since you've listed an instance method, not a static method; if it were static, the first argument would be null). You haven't shown a variable referring to the instance anywhere, so I called it yourInstance in the above.

answered Jan 21, 2013 at 17:16

T.J. Crowder

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Jon Skeet · Accepted Answer · 2013-01-21 17:17:29Z

13

You're currently passing the value of myBytes into the method - as if it's the target object you'd be calling it on. It's the return value.

You want:

byte[] myBytes = (byte[]) m.invoke(target);

where target is the instance you want to call it on (as it's an instance method).

If you don't have an instance, the method will have to be a static method, at which point you'd change the invocation to:

byte[] myBytes = (byte[]) m.invoke(null);

answered Jan 21, 2013 at 17:17

Jon Skeet

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Reto Koradi · Accepted Answer · 2014-11-30 17:11:34Z

0

invoke method takes first argument as instance object for the method followed by parameters. Here method signature is public byte[] myMethod()

Code to invoke myMethod:

byte[] returnVal = (byte[]) m.invoke(MyClass.newInstance());

edited Nov 30, 2014 at 17:11

Reto Koradi

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answered Nov 30, 2014 at 16:39

Bimales Mandal

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